The present disclosure relates generally to image sensors and, more particularly, to complementary metal-oxide semiconductor (CMOS) image sensors (CIS).
CMOS image sensors are widely used in various applications such as digital camera applications. They are used for sensing a volume of exposed light projected towards a semiconductor substrate. To do this, the image sensors use an array of pixels, or image sensor elements, to collect photo energy to convert images into electrical signals that can be used in a suitable application. A CIS pixel includes a photodetector such as a photodiode, photogate detector, or phototransistor, to collect photo energy.
One form of CIS, backside-illuminated (BSI) sensors, sense a volume of light projected towards the backside surface of the substrate of the sensor by using pixels located on the front side of the substrate. The substrate must be thin enough that light projected towards the backside of the substrate can reach the pixels. Backside-illuminated sensors are advantageous in that they provide higher fill factor and reduced destructive interference. A thin substrate is required to reduce crosstalk. However, a thin substrate also degrades the quantum efficiency of the image sensor. The quantum efficiency includes the capability of the sensor to convert light into a signal (including the response of the sensor to different wavelengths of light). In particular, the quantum efficiency of a backside-illuminated sensor may be compromised for light having a long wavelength. For instance, the quantum efficiency may decrease 20-50% for red light.
There are further disadvantages to conventional CIS, including BSI sensors. For instance, they may suffer from noise, such as “kTC noise” (the nomenclature taken from the equation used to calculate the noise level). The noise level includes a measurement of the accuracy with which the incident light can be measured. The noise level is dependent on the type of photodetector on the CIS. A CIS with a photogate type photodetector may have lower noise than a CIS with a photodiode type photodetector. However, a conventional CIS with a photogate type photodetector can suffer from degradations in sensitivity, as incident light is not fully captured by the sensor.
As such, an improved backside-illuminated image sensor is desired.